In large industrial motors and generators, the armature windings, also known as stator windings, are inspected from time to time to confirm that they are operating properly. Each armature winding includes a conductive armature bar(s) wrapped in layers of insulation. The insulation confines the voltage in the bars to prevent arcing between windings, and shields the bars against stray objects that could electrically short the bars and to protect people and equipment. In view of the high voltage levels that are present in industrial generators and motors, the insulation on armature bars must provide an effective and complete barrier surrounding the bars. If the insulating properties of the insulation degrades because it becomes damp or for other reasons, then voltage arcs may jump from the armature bars through degraded regions of the insulation to cause electrical shorts that can harm people and damage equipment.
The insulation on armature windings are inspected from time to time to determine whether the insulation has degraded and, if so, to what extent. The dielectric constant of an insulator provides a: measure of its insulating properties. Accordingly, the insulation of an armature winding can be inspected by determining the dielectric constant of the insulation. The dielectric constant of the insulator can be calculated by measuring the capacitance of the insulation on the armature bars. The dielectric constant indicates such conditions as the amount of dampness in the insulator. A damp insulator may indicate that leak in the water passages in a water cooled armature. A damp insulator may be water damaged and not functioning as an effective insulator.
All insulating materials have a dielectric constant, which is a measure of the amount of energy the insulating material stores when a voltage is applied across the material. The dielectric constant for air is 1.0, for Micapal, a common insulation material for armature windings, is approximately 4, and 80 for water. Because of the large difference in the dielectric constants for Micapal (and other winding insulators) and water, the dielectric constant changes relatively dramatically when an insulator for a winding becomes damp. Accordingly, measuring the dielectric constant of an insulator provides an effective means for detecting water logged insulation on stator windings.
The dielectric constant for an armature insulator can be calculated using capacitance values measured across the insulator. Capacitance and the dielectric constant are related as described in the following equation:
C=kDA/t PA1 D=dielectric constant of the insulation PA1 A=area of the probe electrode PA1 t=thickness of the insulation
Where:
Because the area (A) of a probe electrode and thickness (t) of the insulator are know quantities and the capacitance (C) of the insulator is a measured quantity, the dielectric constant (D) can be relatively easily calculated with the above equation.
The capacitance of the insulation of an armature winding is usually measured at the ends of the windings. Armature windings are generally mounted in longitudinal slots in a cylindrical stator and the windings have ends that extend out from both ends of the stator. At their ends, the armature windings are relatively exposed and accessible to measurement probes. However, there are generally a large number of armature windings that are tightly nested together at the ends of the stator. The gap between adjacent armature windings is often narrow, and the gap can be as small as 0.25 inch or smaller. Because of the narrow gap between armature windings, it has for a long time been very difficult to insert capacitance probes between the windings. In addition, the narrow gaps make it difficult for the probe to form a good electrical surface contact with the surface of the insulation. Without a good surface contact, air in gaps between the probe and the surface of the insulation can distort the capacitance measurement. Accordingly, the narrow gap between armature windings poses long-standing problems for capacitance probes.
In the past, capacitance probes have been formed from foam pads wrapped in metal foil, where the foil is used as an electrode to measure the capacitance of the armature insulation. The wrapped foam pad is attached to a paddle that is used to hold the pad against the armature insulation. The large relative thickness of the foam pad and paddle probe limits this technique to situations where there is a relatively large gap between adjacent armature windings. Alternatively, electrodes for capacitance probes have been formed of adhesive metal tape that adheres to the armature insulation. However, even with metal tape a wide gap between windings is needed for the technician to apply and remove the metal tape. The metal tape has an additional disadvantage of tearing and leaving metal strips on the surface of the winding that can cause a short in the motor or generator. Another probe has been proposed that is comprised of a brass plate electrode having an inclined plane back surface that slides up a matching incline plane in the probe. Before the inclined planes slide over one another, the probe is relatively narrow and is inserted between armature windings. The brass electrode is pressed against the surface of the armature insulation by sliding one of the inclined planes against the other plane to expand the thickness of the probe. This inclined plane electrode also does not fit in the small gaps between many of the electrodes in current stators. Accordingly, there has been a long felt need for a capacitance test device having a probe that can slide between the narrow gap between the armature windings and establish good surface contact with the insulation.
To satisfy the need for a thin capacitance probe, the current invention was developed to measure the capacitance of the insulation wrap on armature bars in electromagnetic generators and motors. The test device comprises a probe having an electrode plate mounted on an inflatable bladder. The plate is formed of an elastomeric conductive material and is sufficiently flexible to make good surface contact with the insulation on an armature bar. The plate is electrically connected to a capacitance meter that is also coupled to ground as are the armature bars during testing. The bladder is initially deflated to reduce the thickness of the test device so that the device can be slid into the narrow gap between the insulated armature bars of a stator. Once positioned between the bars, the bladder is inflated with a pressure hose to press the electrode plate firmly against the surface of the insulation of the armature bar. Once the capacitance of the insulation has been measured, the bladder can be deflated with the aid of a vacuum pump to minimize the thickness of the probe.
It is an object of the current invention to measure the capacitance of the insulation on armature bars in electromagnetic motors and generators. It is a further object of the invention to provide a probe for measuring capacitance that is initially thin for insertion between closely spaced stator windings and expandable to provide a good electrical connection between the probe and the insulation being measured. It is another object of the invention for the probe to be easy to use, reliable in its operation and resistive to oils and other chemicals that commonly exist on and near operating electromagnetic machines. These objects and more have been satisfied with the current invention which is described in detail in the following detailed written description and attached drawings.